1. Field of the Invention
This invention relates generally to a replacement hip implant assembly, and more specifically to an acetabular cup having a plurality of tapered holes therein for receiving polyaxial locking screws and coupling elements, such that the screws may be inserted into the adjacent bone at a wide range of angles.
2. Description of the Prior Art
Hip implant devices, which are utilized to replace corrsponding degenerated joints, generally include an outer metal cup (acetabular cup), and inner polymer or ceramic cup, a metal or ceramic ball, and a femoral post. The outer metal cup is implanted into the acetabulum (the socket portion of the pelvis), and secured therein by a variety of means, some of which are described hereinbelow. The inner polymer or ceramic cup seats within the outer metal cup and has a smooth semi-spherical interior conformation for receiving the metal or ceramic ball. The ball is smooth so as to provide low-friction polyaxially rotatability within the semi-spherical interior of the inner cup. The ball is also coupled to the top of the femoral post, which extends into the top of the femur to complete the coupling of the hip to the femur, while permitting rotation of the artifial joint.
There are a number of prosthetic hip devices which have been disclosed in the art which include the elements set forth above in various embodiments. The various embodiments disclose a spectrum of means for securing the acetabular cup to the pelvis. Some teach integral projections extending from the exterior surface of the cup-shaped body which provide anchoring for the element within the surrounding bone. European Patent Application No. 13,863, published on Jun. 8, 1980 describes such a device. The cup of this reference is anchored to the bone by a central pin and a number of other pins distributed over the surface of the cup. The pins have sawtooth notches along their length and are inserted into pre-drilled holes in the bone. Similarly, European Patent Application No. 211,169 published Feb. 25, 1987, discloses an acetabular cup which includes an external boss protruding from the outer surface of the cup to fit into a pre-drilled hole in the acetabulum.
An advance over fixed and integral anchoring projections is illustrated in U.S. Pat. No. 4,828,565 wherein the acetabular shell includes a pair of outwardly projecting elements which are fixed, but through which screws may be passed to further secure the cup in the bone.
The use of screws alone to anchor the acetabular cup in the surrounding bone is described in U.S. Pat. Nos. 3,939,497 and 4,685,923 in which a series of radially arranged pegs may be selectively projected outwardly through the wall of the cup into the surrounding bone.
In each of the devices described above, the integral anchoring projections and/or the selectively introduceable screws, the entrance angles of the securing means are rigidly predetermined. As it is desirable to provide the surgeon with the freedom to choose the entrance angle of the screw to best suit the individual conformation of the surrounding bone on a patient-by-patient basis, U.S. Pat. No. 4,792,337 provides an acetabular cup having countersunk holes and screws having rounded heads, such that the orientation of the screws may be varied with respect to the cup and each other.
While it is a substantial gain over the previous designs to provide for variability in screw entrance angle, a common concern with screws being implanted into bone is that the screws may dislocate and begin a process known as "pull-out". This is an especially important problem in conjunction with acetabular cups, wherein dislocated screws often impede the rotational motion of the device and/or provide a wearing impetus to erode the low friction surfaces at the motion interface.
There are no presently available screw plate assemblies which present a flush surface and provide for means of preventing both screw pull-out from the bone and screw backout, while simultaneously providing for a wide range of angulation for the bone screws.
It is therefore, an object of the present invention to provide a new and novel acetabular cup and anchoring screw design which allows for polyaxial entrance of the screw into the surrounding bone.
It is also an object of the present invention to provide an orthopaedic implant which has a simple and effective mechanism for locking the bone screw to the acetabular cup.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.